fix bugs for 5GSMobilityIdentity decoding

src/nas/ies/5GSMobilityIdentity.cpp

@@ -83,7 +83,7 @@ int _5GSMobilityIdentity::_5g_s_tmsi_encode2buffer(uint8_t* buf, int len) {
 
 //------------------------------------------------------------------------------
 int _5GSMobilityIdentity::_5g_s_tmsi_decodefrombuffer(uint8_t* buf, int len) {
-  int decoded_size = 0;
+  int decoded_size = 1;
   _5g_s_tmsi       = (_5G_S_TMSI_t*) calloc(1, sizeof(_5G_S_TMSI_t));
   uint8_t octet    = *(buf + decoded_size);
   decoded_size++;